Prostaglandin H synthase is a microsomal glycoprotein which exhibits two distinct catalytic activities: a fatty acid oxygenase and peroxidase. The fatty acid oxygenase (cyclooxygenase) reaction is the first committed step in the biosynthesis of prostaglandin (PG) D2, PGE2, PGF2, PGI2 and thromboxane A2, which collectively make up the bulk of eicosanoids. As for other fatty acid oxygenases, the cyclooxygenase requires hydroperoxide for catalytic activity. The eicosanoids derived from PGH2 are involved in a wide variety of physiological and pathological processes including inlammation, immune surveillance, sleep induction, stroke, and myocardial infarction. The overall goal is to determine the reaction mechanisms of the cyclooxygenase and peroxidase activities of PGH synthase, and the implications of these mechanisms on the regulation of eicosanoid biosynthesis. The specific aims are to: a) identify and quantitate the redox centers in the synthase and establish their roles in catalysis; b) determine the mechanism of activation of the cyclooxygenase by hydroperoxide; c) determine the basis for the influence of fatty acid structure on the hydroperoxide activator requirement of the cyclooxygenase; d) formulate a mechanism for the inactivation of the cyclooxygenase and peroxidase activities during catalysis; and e) characterize the subunit structure and the physical arrangement of the subunits in the synthase, and their influence on catalytic activity. The methodologies to be employed include: analysis of the cyclooxygenase and peroxidase activities; protein and polypeptide purification and characterization: proteolytic digestions and fragment analysis; metal analysis; UV-vis spectrophotometry; magnetic circular dichroism; and electron paramagnetic resonance.